Radiation of frequencies with wavelengths much larger than the space available for an antenna is often necessary. This results in the use of an electrically small antenna. An electrically small antenna has a very small bandwidth; therefore, small perturbations in the shape of the antenna or in surrounding object movement results in mistuning of the antenna to the extent that it does not radiate electromagnetic energy.
In transmitting antennas which have a narrow tuned bandwidth, it is important to tune the antenna to the transmitted frequency in order to achieve optimum performance in which maximum power is radiated. If the antenna environment changes during the period of a transmission, it is necessary to dynamically adjust the antenna and the antenna matching circuits.
Prior to the present invention, many systems for automatically tuning transmitting antennas were known. In some of these systems, the antenna is automatically tuned to the transmitted frequency using a forward and reflected power measurement. This method enables locating the measurement instruments remotely from the actual radiating element; however, it is difficult to determine if the antenna is above or below resonance. This is because the loop antenna, commonly used, normally returns an inductive reactance from both above and below resonance when only an antenna bandwidth or two in frequency away from the antenna resonance frequency. Additionally, the transmission line separating the antenna from the measurement point transforms the antenna impedance observed by the measuring circuit complicating the determination of the feedpoint impedance.
Prior antenna systems have also used a phase discriminator in the antenna feed line either at the transmitter or at the antenna. Again, as an electrically small loop antenna will not necessarily present a feedpoint capacitive reactance component when tuned off resonance, it is not possible to determine if the loop tuning capacitor should be increased or decreased to achieve antenna resonance.
Not being able to readily determine the radiating element complex impedance means that closing a simple null seeking feedback loop cannot be implemented for resonant frequency tuning and subsequent tracking with movement of the antenna or the surroundings of the antenna.
Additionally, prior art tuning of electrically small loop antennas is accomplished with a mechanical capacitor that is tuned by a rotating actuator such as a direct current motor or a stepping motor. This results in a system that is bulky, heavy and uses considerable primary power.